Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

core/os/spin_lock.h

@@ -48,4 +48,5 @@ public:
 		locked.clear(std::memory_order_release);
 	}
 };
+
 #endif // SPIN_LOCK_H

core/os/thread.h

@@ -123,4 +123,5 @@ public:
 };
 
 #endif // THREAD_H
+
 #endif // PLATFORM_THREAD_OVERRIDE